Method for producing circuit boards with deposited metal patterns and circuit boards produced thereby

ABSTRACT

A process for producing circuit boards involves the coating of a resinous substrate with a fluid mixture of an epoxy polymer component and a rubber polymer which is interactive therewith at temperatures of at least 180° F. Preferably the rubber is a low molecular weight polyfunctional reactive butadiene/acrylonitrile interpolymer which is terminated by vinyl or carboxyl functional groups. The rubber component comprises at least 70 percent by weight of the epoxy polymer component, and the coating has a thickness of 0.001-0.015 inch. The coating is partially cured to effect partial polymerization of the epoxy prepolymer, further polymerization of the rubber, if it is low molecular weight, and interaction of the rubber and epoxy polymer to form a matrix of the interacted rubber/epoxy. The exposed surface of this coating is then etched, and metal is deposited on the surface to form a conductive layer. A conductive pattern is formed therein, and heat and pressure are then applied to the conductive pattern and coating to fully cure the coating and bond the coating to the metal layer, and thus the conductive pattern to the resinous substrate. The metal layer may be deposited chemically or by vacuum metallizing and like techniques.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation-in-part of application Ser.No. 06/909,256 filed Sept. 19, 1986, now U.S. Pat. No. 4,707,394 Nov.17, 1987.

BACKGROUND OF THE INVENTION

The present invention relates to circuit boards, and more particularlyto a novel process for producing circuit boards in which a conductivelayer of metal forms a circuit pattern upon, and is firmly bonded to, aresinous substrate providing the support therefor, and to the welldefined and bonded circuit boards which are produced thereby.

Various techniques have been utilized for generating printed circuitboards, including the lamination of foil to resinous or ceramicsubstrates and etching of the circuit pattern, the chemical depositionof the metal pattern from a plating solution, and the deposition of aconductive metal layer or pattern by vacuum metallization, sputtering,and like techniques, followed by electrodeposition of the thicker metaldeposit of the circuit pattern.

Traditional copper-clad boards are manufactured using multiple layers ofepoxy resin impregnated glass cloth which are pressed under heat andpressure together with copper foil to form a copper-clad laminate. Toenhance the adhesion of the epoxy/glass prepreg to the copper, thecontact surface of the copper is treated so as to form a rough oxidesurface. The ultimate peel strength is defined both by mechanicaladhesion of the reflowing epoxy during the pressing phase as it flowsinto the microrough oxide surface of the copper foil and also by thechemical adhesion of the epoxy resin to the copper oxide. Thesecopper-clad boards are then used for the manufacture of printed circuitboards using subtractive processing, i.e., the removal of the copperbetween the desired circuit elements of the conductive pattern.

In an effort to obtain fine patterns with well defined edges, industryhas been decreasing the thickness of the metal layer, and problems ofobtaining good bond between the metal layer and the underlying substratehave been increasing. Thus, where line widths and spacings were commonly0.008-0.015 inch several years ago, currently demands are being made forlines width and spacings of 0.003-0.007 inch and sometimes as little as0.001 inch.

The smaller the width and the spacing, the thinner the metal layer mustbe for sharp definition. As is known, the thicker the copper deposit tobe etched, the greater the extent of the etch-back. Therefore, toachieve fine lines, it is necessary to have a thin layer of copper toetch; the finer the desired lines, the thinner the copper. Theproduction of foil laminated boards using an extremely thin copper layernecessary is impractical so that industry has turned to additive andsemi-additive processing, i.e., the deposition of metal onto thesubstrate.

The main problem in such additive processing is one of the adhesion ofthe copper layer deposited onto a cured epoxy laminate. Because adhesionis not reversible, the peel strength will not be the same as thatachieved in a foil/board laminate.

Many techniques and methods have been tried to overcome this deficiency,but none has fully overcome it in a practical, production-efficient way.Among the techniques which have been proposed to improve the bonding orpeel strength between the metal layer and the underlying substrate areapplication of intermediate "adhesive" coatings and etching orroughening of the substrate before application of the metal layer, etc.Generally, these techniques have effected improvements in bond strength,but they have not produced uniformly good bonding and/or have involvedsubstantial additional expense and processing difficulties.

It is an object of the present invention to provide a novel additiveprocess for the production of circuit boards in which a thin metal layeris deposited upon and firmly bonded to an underlying resinous substrate.

It is also an object to provide such a process which inherently lendsitself to the fabrication of miniaturized circuitry with good adhesionof the individual circuit portions to the underlying substrate.

Another object is to provide such a process which may be practicedrelatively economically and expeditiously.

A further object is to provide circuit boards utilizing relatively thinmetal deposits of relatively narrow width and close spacing to form theconductive pattern, and in which the conductive pattern is firmly bondedto a resinous substrate.

SUMMARY OF THE INVENTION

It has now been found that the foregoing and related objects can bereadily attained in a process for producing circuit boards having ametallized circuit pattern firmly bonded to a resinous substrate, inwhich a resinous substrate is coated with a fluid mixture of aformulation containing an epoxy polymer component and a rubber componentwhich is interactive therewith at temperatures of at least 180° F. Therubber component comprises 50-200 percent by weight of the epoxy polymercomponent, and the coating has a thickness of 0.0005-0.015 inch.

This coating is partially cured to produce partial cross-linking of theepoxy polymer component, and interaction of the rubber and the epoxypolymer. The exposed surface of the coating is etched to produce amicroporous surface. Copper or other suitable metal is then deposited onthe surface of the coating to form a conductive layer withmicroformations extending into the recesses of the microporous surface.Metal is then deposited thereon to form a conductive pattern on thecopper layer. Heat and pressure are applied to the coating and to themetal deposit either before or after the electrodeposition of the metalto provide the conductive pattern to fully cure the coating, therebyfirmly bonding the coating to the metal layer and thereby the metalpattern through the coating, to the resinous substrate.

Preferably, the rubber interpolymer is a low molecular weightpolyfunctional reactive rubber component comprising an interpolymer ofbutadiene and acrylonitrile which is terminated by functional groupsselected from vinyl and carboxyl. It has a Brookfield viscosity of100,000-400,000 cps at 27° C. and preferably 120,000-300,000. Thefunctional terminal groups interact with the epoxy polymer componentduring further polymerization thereof

Desirably, the etching step utilizes a chemical etchant to attack theexposed surface and produce the microporous characteristic.Alternatively, the etching step may utilize a plasma to attack theexposed surface of the coating and produce the microporouscharacteristic.

In the preferred embodiment, the metal layer is deposited by a processof vacuum metallizing. Alternatively, the metal layer may be depositedby electroless chemical deposition from a plating solution, requiringpreliminary surface activation of the coating.

The preferred coating compositions utilize a rubber component in whichthe terminal functional group is a vinyl group, and the rubber componentcomprises 100-200% by weight of the epoxy polymer component.

The step of partially curing the coating comprises developing atemperature of 250-350° F. in the coating for a period of at least 2hours, and the step of fully curing the coating comprises developing atemperature in the coating of 280°-350° F. for a period of at least 2hours. In the preferred process, the step of applying heat and pressureto finally cure the coating at least partially embeds the conductivepattern in the coating. Generally, a continuous layer of copper or othermetal is initially deposited on the coating and thereafter selectivelyetched to remove the layer between the elements of the conductivepattern.

The final curing of the coating may be effected either before theelectrodeposition of the metal to provide the full thickness of theconductive pattern or preferably thereafter to achieve increasedmechanical adhesion by embedding in the coating at least a portion ofthe electrodeposited metal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmentary cross sectional view of a circuit board assemblyduring one point during the process of manufacture of the presentinvention with the components drawn to a greatly enlarged and somewhatdistorted scale;

FIG. 2 is a similar view at a subsequent stage in the process ofmanufacture of the circuit board; and

FIG. 3 is a similar view of the circuit board assembly at the finalstage of manufacture.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As has been previously indicated, the method of the present inventioninvolves a series of steps to produce a final circuit board structure inwhich the conductive metal pattern is firmly bonded to the underlyingsubstrate through an intermediate coating. In this process, generallythe following steps are involved.

Initially, the substrate which will conventionally be a fiber-filledepoxy or other resinous substrate is coated with the special coatingformulation of the present invention, and this coating is partiallycured. The exposed surface of the coating is then etched to providemicroporous surface characteristics, and a layer of metal is depositedon the surface of the coating. A thicker deposit of metal is formedthereon in the desired conductive pattern. Heat and pressure are appliedto the conductive layer or pattern and the coating to fully cure thecoating, to firmly bond the coating to the metal layer, and thereby tobond the metal pattern through the coating to the substrate.

The present invention utilizes conventional substrates to provide thestrength and dimensional stability of the assembly circuit board.Generally, glass filled epoxy resin or other reinforced resinousmaterials are employed for such substrates. It is also possible to useceramic substrates.

As is also well known, the conductive circuit may be formed on one orboth surfaces of the substrate. Generally, this conductive circuit isformed of copper although other conductive metals may be employedincluding gold, silver, palladium, aluminum, and alloys thereof,depending upon the characteristics desired in the metal pattern. Varioustechniques may be utilized for forming the conductive layer includingchemical plating or electroless deposition from a solution of the metalto be deposited, vacuum metallizing, sputtering, and the like. Of thesevarious techniques, vacuum metallizing and electroless deposition aregenerally preferred. Where electroless deposition is utilized, it isgenerally necessary to pretreat the surface upon which the deposit is tobe made so as to catalytically activate it and cause the metal ions toplate from the solution.

The intermediate coating which will bond strongly to the copper or otherdeposited metal and also to the underlying substrate comprises a fluidcomposition at room temperature containing an epoxy polymer componentand rubber polymer which will interact with the epoxy polymer at anelevated temperature of at least 180° F. The rubber component mustcomprise at least 50 percent by weight of the epoxy polymer component,and preferably 100 to 200 percent by weight thereof. This develops acoating having an epoxy-rubber matrix with some epoxy particlesdispersed within it. This is in contrast to other systems which uselower levels of rubber which produce a matrix of epoxy with rubber orepoxy-rubber particles as the second phase of a two-phase material.Moreover, the most desirable compositions are those wherein the rubbercomponent comprises 120 to 200 percent by weight of the epoxyprepolymer.

The preferred rubber compositions are low molecular weightpolyfunctional butadiene/acrylonitrile interpolymers with reactiveterminal groups. These terminal groups are desirably selected from thegroup consisting of vinyl and carboxyl. Highly beneficial results havebeen obtained by use of those with vinyl groups as the chain terminationand having a vinyl equivalent weight of 700 to 1500, and preferably 900to 1300, and an acrylonitrile content of 14 to 20 percent and preferably15 to 18 percent. The Brookfield viscosity at 27° C. ranges between125,000 and 375,000 cps, and is preferably in the range of 200,000 to300,000 cps. Exemplary of such a rubber component is HYCAR 1300X23 soldby the B.F. Goodrich Company of Cleveland, Ohio.

The other class of rubbers which may be utilized are those which havecarboxyl terminations for the rubber chain. These will normally have anacrylonitrile content of 15 to 22 percent and preferably 16 to 20percent, and a carboxyl content defined by an acid number of 25 to 35and preferably 27 to 31 (or a carboxyl equivalent per hundred of about0.045-0.060). These rubbers will generally have a Brookfield viscosityat 27° C., of 100,000 to 200,000 cps and preferably 110,000 to 160,000cps. Exemplary of such rubbers are those sold by B.F. Goodrich Companyunder the designation HYCAR 1300X8. Goodrich further defines these ashaving a molecular weight (Mn) of 3600 and a functionality of 1.8.

Utilizing the preferred vinyl-terminated rubbers, the rubber content maydesirably range from 100 to 200 percent of the epoxy polymer component.Utilizing the carboxyl terminated rubbers, the rubber content preferablycomprises 50 to 120 percent by weight of the epoxy polymer component.

Other materials may be incorporated in the coating formulation in orderto provide desirable characteristics including solvents to controlviscosity or to assist dissolution of other components. Suitablesolvents include acetone, vinyl toluene, styrene, methyl ethyl ketone,methylene chloride, methyl isobutyl ketone and ethyl glycol acetate.Glycol ether may also be used in some formulations. Fire retardants suchas antimony oxide may be included, as can be various bromine compounds.Fillers such as silica can be incorporated to adjust viscosity.

High temperature curing agents for the epoxy component which provide theopportunity for two-stage curing are incorporated in the composition.Dicyandiamide has proven particularly effective, and it is readilydissolved in dimethyl formamide. Tertiary amines such asbenzyldimethylamine are useful as accelerators.

The epoxy polymers are typically the reaction products ofepichlorohydrin and bis-phenol A. In some instances, the reactionproduct may be brominated to provide inherent flame retardance. Mixturesof epoxy polymer systems may be employed so long as they are compatible,and these may desirably be utilized to produce a balance of propertiesincluding fire retardance. Typically, the epoxy polymer component willhave an epoxide equivalent weight of about 410 to 480 and preferablyabout 415 to 450.

The viscosity of the coating composition should be adjusted dependingupon the coating method to be used and the thickness desired. Generally,it will fall within the range of 3 to 60 seconds, and preferably 5 to 50seconds, as measured with the Zahn Cup using an orifice of 0.125 inchdiameter. For dip coating, the desirable flowout time for double sidedcircuit boards is approximately 15 to 25 seconds as so measured, and thedesirable flowout time for multilayered boards is about 6 to 12 secondsas measured by the Zahn Cup.

The coatings may be applied by any suitable technique, and dip androller coating are simple and highly effective. The coating should havea thickness of 0.0005-0.015 inch, and preferably about 0.0007-0.005inch.

Following application of the coating to the substrate, an initialpartial cure is effected to produce further polymerization of a lowmolecular weight rubber component if employed, to produce partialcross-linking of the epoxide, and to produce interaction of the epoxidewith the rubber component. Generally, this will involve bringing thecoating (and the underlying substrate) to a temperature of about230°-300° F. for a period of 2 to 5 hours and preferably 240°-260° F.for a period of about 3 to 5 hours. Obviously, the process conditionsshould be selected to ensure that the coating is not fully cured and sothat some degree of thermoplasticity remains.

Following the partial curing of the applied coating, the exposed surfaceshould be etched to produce a microporous surface characteristic.Preferably and most conveniently, the etching is accomplished by achemical etching process using oxidants such as potassium dichromate,sulfuric acid, and mixtures thereof. Alternatively, techniques such asexposure to a plasma of oxygen, ammonia or nitrogen will effect thedesired roughening of the surface to produce the microporouscharacteristic. Other techniques which can be considered are mechanicalabrasion, dispersion of readily etchable or leachable components in thecoating material (and etching after its application), application of apreroughened release film onto the surface of the coating prior to itsinitial curing to emboss it, etc.

Following etching of the coating to provide the microporouscharacteristic, the conductive metal layer is deposited thereon (i)either as a uniform layer over the entire surface in which case it issubsequently etched away to leave the desired pattern, or (ii) in apattern by providing a suitable resist to block the deposition thereof.As has been previously indicated, various techniques may be used toeffect the deposition including vacuum metallizing and electrolesschemical deposition which are the preferred techniques. Electrolessdeposition will require initial activation of the surface. Theconductive metal layer will normally utilize these techniques to providea thin conductive coating of about 10-100×10⁻⁶ inch; and preferablyabout 50-70×10⁻⁶ inch.

Because the metal layer is being deposited upon a microporous surface,the deposit will follow that surface at the interface and thus exhibitmicroroughness at the interface although it will be relatively smooth onits exposed face.

A photoresist pattern is generally formed thereon, and then the fullthickness of the desired conductive metal pattern is developed byconventional electroplating techniques. Generally, the final thicknessof the pattern will be on the order of 1.0-30.0×10⁻⁴ inch, and usuallyabout 5.0-15.0×10⁻⁴.

After the conductive pattern has been fully formed by electroplating,the photoresist is removed, and the original thin metal layer is etchedaway between the conductive elements to expose the surface of thecoating.

In accordance with the preferred process, the circuit board is subjectedto a final curing operation wherein it is placed under heat and pressureto effect final curing of the coating. Generally, the temperaturesrequired will be on the order of 280°-350° F. for a period of about 2 to6 hours, and the pressure will normally be on the order of 1.0-300p.s.i.g. The pressure is conveniently effected by placing the boardbetween polished metal platens which may also be heated to effect thedesired temperature. In this curing step, the coating composition flowsinto the microrough surface of the original metal layer of the circuitpattern to produce intimate surface contact therebetween. Thus, thecoating bonds to the conductive pattern in a fashion similar to thatachieved in foil/board laminating processes.

Alternatively, the partially processed circuit board may be subjected tothe final curing step for the coating before the electrodeposition ofthe full thickness of the metal to provide the conductive pattern. Theheat and pressure smooth out the exposed surface of the metal layer andat least partially embed it within the coating. This may leave smalldiscontinuities in the metal layer which are readily filled in duringthe electrodeposition step. This earlier curing step appears to reducethe greater advantages mechanical adhesion achieved by embedment of aportion of the thickness of the electrodeposit in the coating, but itdoes appear to result in a smoother surface on the electrodepositedmetal.

The exact nature of the interaction between the cross-linking epoxypolymer and rubber polymer is not fully understood. In using thepreferred low molecular weight polyfunctional rubbers, a true reactionbetween functional groups is believed to occur. When using a highmolecular weight rubber, some interaction takes place, and this may beadditive at double bonds along the rubber chain, or at pendant groupsand cross-linking sites. Whatever the case, the process is believed toproduce a "bonding" of the epoxy to the rubber to form a product whichcomprises a matrix thereof in which is dispersed some epoxy polymer.Upon etching, the rubber component at the surface is still attacked bythe etchant to provide the microporosity.

Depending upon the amount of polymerization in the initial curing step,and the temperature and pressure employed in the final curing step, theconductive pattern may be embedded into the coating, either in part orfully to produce a flush circuit. Desirably, the pressure is sufficientto embed the conductive pattern an amount equal to at least 10 percentof its thickness. This is believed to enhance adhesion by reason of thegreater surface area in contact between the coating and the metalpattern. Whatever the case, the microporous characteristic on thesurface of the coating at the time of deposition of the initial portionof the metallic layer, and the subsequent curing step produce anextremely high degree of adhesion between the metallic layer and thecoating and thereby through the coating to the substrate. Peel strengthsof 10-20 pounds per inch are readily and customarily attainable.

As indicated hereinbefore, the substrate may be coated on both surfacesand conductive patterns formed on both surfaces of the substrate.Moreover, multiple boards may be assembled and bonded to produce amultilayer assembly.

As previously indicated, various metals may be used in the presentprocess. Moreover, a strike or very thin coating of a first metal, suchas chromium and titanium, may be used to improve metal bonding, and thena layer of a second metal such as copper deposited thereon. This isfollowed by the deposition of copper or some other metal to develop thedesired thickness for the conductive pattern in the circuit board.

Diagrammatically illustrative of the process as hereinbefore describedis the attached drawing. In FIG. 1, the substrate is indicated by thenumeral 10, the coating by the numeral 12, and the initial metal layerby the numeral 14. As seen in FIG. 2, a photoresist pattern 16 isdeveloped on the surface of the conductive layer 14, and theelectroplated, relatively thick metal deposit on the layer 14 isdesignated by the numeral 18. Turning now to FIG. 3, the conductivepattern 18/14 following removal of the resist 16 and etching of thelayer 14 between the conductive pads 18 is partially embedded into theresin of the coating 12 by the application of heat and pressure.

Exemplary of the present invention are the following specific exampleswherein all parts are parts by weight unless otherwise indicated.

EXAMPLE ONE

The base material or substrate is a general purpose rated epoxy/glasslaminate approximately 0.060 inch in thickness. A coating formulation ofthe following composition is dip coated to a thickness of 2.0-2.5×10⁻³inch:

Epoxy resin: 70.0

Dicyandiamide: 2.0

Benzyldimethylamine: 0.5

Rubber (HYCAR 1300X23 Goodrich): 110.0

Silica: 10.0

Glycol ether: 42.0

Acetone: 53.0

Dimethylformamide: 8.0

After application, the coating is partially cured at 250° F. for aperiod of 4 hours in a forced air oven.

The base material with the partially cured coating is then subjected toan etching treatment to render the surface microporous by an etchingsolution containing potassium dichromate, sulfuric acid, orthophosphoricacid, and water. To achieve the desired degree of etching, the coatedbase material is immersed in the etching solution for 3.5 minutes withthe solution having a temperature of 40° C. This process step removespart of the acrylonitrile rubber from the surface of the coating.

After drying, a thin coating of copper is then applied to the coatedbase material by vacuum metallizing to a thickness of 50-70×10⁻⁶ inch. Aphotoresist is then exposed and developed to produce the desired circuitpattern, following which the pattern is plated to the required copperthickness of approximately 1 oz. per square foot. The resist is thenremoved and the thin, initial copper layer is removed by etching in aferric chloride solution.

At this stage, the board is placed under contact pressure between twosteel plates and placed in an oven at 290° F. for a period of about 4hours. During this period, the partially cured coating reflows and bondsto the copper circuit pattern so that the final bond is effected by theadhesion of the reflowing coating to the micro-roughened initial copperlayer to produce a peel strength of about 12 lb/in.

EXAMPLE TWO

After preparation of the base material in accordance with Example One, acoating the following formulation is applied:

Epoxy resin: 70.0

Dicyandiamide: 2.0

Benzyl dimethylamine: 0.5

Rubber (HYCAR 1300X8 Goodrich): 45.0

Silica: 10.0

Glycol ether: 67.0

Dimethylformamide: 8.0

Initial curing of this system is effected at 290° F. for a period of 4hours, and the final cure is effected for 5 hours at 300° F.

The remaining steps of Example One are repeated.

EXAMPLE THREE

An FR-4 type epoxy/glass substrate is used and the coating in theprocess of Example One has the following formula to retain the firerating:

Epoxy resin: 10.0

Brominated epoxy resin: 62.5

Dicyandiamide: 2.0

Benzyl dimethylamine: 0.5

Rubber (HYCAR 1300X23 Goodrich): 115.0

Silica: 10.0

Antimony oxide: 3.0

Acetone: 42.0

Dimethyl formamide; 8.0

EXAMPLE FOUR

In a modification of the process described in the preceding examples,the final curing step is effected after the deposition of the initialmetal layer and before electrodeposition of the metal to provide thefull thickness of the pattern.

The process as set forth in Example One is substantially repeated exceptthat the coating has a thickness of about 1.5-2.0×10⁻³ inch, and thefinal curing step is effected after the formation of the vacuummetallized deposit by subjecting the partially processed board to atemperature of 280° F. for 3 hours at 250 p.s.i. After the applicationof the heat and pressure in this step, the vacuum metallized depositappears to be substantially embedded in the coating.

After electroplating, the surface of the conductive pattern appearssmooth, and the peel strength is found to be 12.6 lb/in.

Thus, it can be seen from the foregoing detailed specification andexamples, that the process of the present invention provides arelatively simple but highly effective technique for achieving goodbonding between a deposited metal layer and an underlying resinoussubstrate. Peel strengths of 10-20 lb/in. are readily and uniformlyattainable due to the excellent adhesion afforded by the coating whichmay be characterized as an epoxy filled and modified rubber polymer.Good line definition and close spacing can be readily attained due tothe ease with which the process may be practiced and the bond strengthswhich may be attained.

Having thus described the invention, what is claimed is:
 1. In a processfor producing circuit boards having a metallized circuit pattern firmlybonded to a resinous substrate, the steps comprising:(a) coating aresinous substrate with a fluid mixture of a formualtion containing;(i)a low molecular weight polyfunctional reactive rubber componentcomprising an interpolymer of butadiene and acrylonitrile; and (ii) anepoxy polymer component, said components being interactive attemperatures of at least 180° F., said rubber component comprising50-200 percent by weight of the epoxy polymer component, said coatinghaving a thickness of 0.0005-0.015 inch; (b) partially curing thecoating to produce partial cross-linking of said epoxy polymercomponent, further polymerization of the rubber, and interaction of saidrubber and epoxy polymer components; (c) etching the exposed surface ofthe coating to produce a microporous surface; (d) depositing metal onthe microporous surface of the coating to form a conductive layer withmicroformations extending into the recesses of said microporous surface;(e) applying heat and pressure to said conductive layer and coating tofully cure said coating, thereby firmly bonding said coating to saidmetal layer and thereby said conductive layer through said coating tosaid resinous substrate; and (f) electrodepositing metal on saidconductive layer to form a conductive pattern therewith, said conductivepattern being bonded firmly to said substrate through said conductivelayer and coating.
 2. The process in accordance with claim 1 whereinsaid rubber interpolymer has terminal functional groups selected fromvinyl and carboxyl.
 3. The process in accordance with claim 1 whereinsaid rubber interpolymer has a Brookfield viscosity of 100-400,000 cpsat 27° C.
 4. The process in accorance with claim 1 wherein saidfunctional terminals groups interact with the epoxy polymer componentduring curing thereof.
 5. The process in accordance with claim 1 whereinsaid etching step utilizes a chemical etchant to attack the exposedsurface of said coating and produce the microporous characteristic. 6.The process in accordance with claim 1 wherein said etching steputilizes a plasma to attack the exposed surface of said coating andproduce the microporous characteristic.
 7. The process in accordancewith claim 1 wherein said metal layer is deposited by a process ofvacuum metallizing.
 8. The process in accordance with claim 1 whereinsaid metal layer is deposited by electroless chemical deposition from ametallic solution.
 9. The process in accordance with claim 1 whereinsaid terminal functional group is a vinyl group and wherein said rubbercomponent comprises 100-200% by weight of the epoxy prepolymercomponent.
 10. The process in accordance with claim 1 wherein said stepor partially curing the coating comprises developing a temperature of250°-350° F. in the coating for a period of at least 2 hours, andwherein said step of fully curing the coating comprises developing atemperature in the coating of 280°-350° F. for a period of at least 2hours.
 11. The process in accordance with claim 1 wherein said step ofapplying heat and pressure at least partially embeds said conductivelayer in the coating.
 12. The process in accordance with claim 1 whereina continuous layer of metal is initially deposited on said coating andthereafter selectively etched to remove said layer between said elementsof said conductive pattern.
 13. In a process for producing circuitboards having a metallized circuit pattern firmly bonded to a resinoussubstrate, the steps comprising:(a) coating a resinous substrate with afluid mixture of a formulation containing a rubber component and anepoxy polymer component which are interactive at temperatures of atleast 180° F., said rubber component comprising 50-200 percent by weightof the epoxy polymer component, said coating having a thickness of0.0005-0.0015 inch; (b) partially curing the coating to produce partialcross-linking of said eopxy polymer component, and interaction of saidrubber component and epoxy polymer components to form a matrix of theinteracted rubber and epoxy components; (c) etching the exposed surfaceof the coating to produce a microporous surface; (d) depositing metal onsaid microporous surface of the coating to form a conductive layer withmicroformations extending into the recesses of said microporous surface;(e) applying heat and pressure to said conductive layer and coating tofully cure said coating, thereby firmly bonding said coating to saidconductive layer and thereby said conductive layer through said coatingto said resinous substrate, said heat and pressure embedding saidconductive layer in said coating; and (f) electrodepositing metal onsaid conductive layer to form a conductive pattern therewith.
 14. Theprocess in accordance with claim 13 wherein said rubber component hasfunctional terminal groups which interact with the epoxy prepolymercomponent during said metal curing step.
 15. The process in accordancewith claim 13 wherein said etching step utilizes a chemical etchant toattack the exposed surface of said coating and produce the microporouscharacteristic.
 16. The process in accordance with claim 13 wherein saida conductive layer is deposited by a process of vacuum metallizing. 17.The process in accordance with claim 13 wherein said rubber component isa low molecular weight polyfunctional reactive rubber comprising ainterpolymer of butadiene and acrylonitrile which is terminated by afunctional vinyl group and wherein said rubber component comprises100-200% by weight of the epoxy polymer component.
 18. The process inaccordance with claim 17 wherein said rubber interpolymer has terminalfunctional groups selected from vinyl and carboxyl.
 19. In a circuitboard, the combination comprising:(a) a resinous substrate; (b) anintermediate coating on one surface of said substrate of about0.0005-0.015 inch and containing a rubber component and an epoxy polymerwhich have interacted to provide a matrix of the interacted rubber/epoxypolymer, said rubber component comprising at least 35 percent by weightof said coating; and (c) a metal conductive pattern on said coatingproviding a circuit, said conductive pattern having a base metal layerwith microformations at the interface with said coating an extendingthereinto, said conductive pattern being embedded in said coating to adepth substantially equal to the thickness of said base metal layer,said coating being firmly bonded to said base metal layer and saidconductive pattern thereby being bonded through said coating and basemetal layer to said substrate.
 20. The circuit board in accordance withclaim 19 wherein said circuit board is produced by a process comprisingthe steps of:(a) coating a resinous substrate with a fluid mixture of aformulation containing a rubber component and an epoxy polymer componentwhich are interactive at temperatues of at least 180° F., said rubbercomponent comprising 50-200 percent by weight of the epoxy polymercomponent in said formulation, said coating having a thickness of0.0005-0.015 inch and containing at least 35 percent by weight of saidrubber component; (b) partially curing the coating to produce partialcross-linking of said epoxy polymer component, and interaction of saidrubber and epoxy polymer components to form a matrix of the interactedrubber and epoxy components; (c) etching the exposed surface of thecoating to produce a microporous surface; (d) depositing metal on saidmicroporous surface of the coating to form a conductive layer withmicroformations extending into the recesses of said microporous surface;(e) applying heat and pressure to said conductive pattern and coating tofully cure said coating, therby firmly bonding said coating to saidconductive layer and thereby said conductive layer through said coatingto said resinous substrate, said heat and pressure embedding saidconductive layer in said coating; and (f) electrodepositing metal onsaid conductive layer to form a conductive pattern therewith.
 21. Thecircuit board of claim 20 wherein said rubber component is a rubberypolymer with terminal functional groups selected from vinyl andcarboxyl.
 22. The circuit board of claim 21 wherein said rubbery polymerhas a Brookfield viscosity of 100-400,000 cps at 27° C.
 23. The cirucitboard of claim 20 wherein said rubber component is a low molecularweight polyfunctional reactive rubber comprising an interpolymer ofbutadiene and acrylonitrile which is terminated by a functional vinylgroup and wherein said rubber component comprises 100-200% by weight ofthe epoxy polymer component.
 24. The circuit board of claim 19 whereinsaid rubber component comprises at least 50 percent by weight of saidcoating.